In the manufacture of semiconductor devices and other ion related products, ion implantation systems are used to impart dopant elements into semiconductor wafers, display panels, or other types of workpieces. Typical ion implantation systems or ion implanters impact a workpiece with an ion beam utilizing a known recipe or process in order to produce n-type or p-type doped regions, or to form passivation layers in the workpiece. When used for doping semiconductors, the ion implantation system injects selected ion species to produce the desired extrinsic material. Typically, dopant atoms or molecules are ionized and isolated, sometimes accelerated or decelerated, formed into a beam, and implanted into a workpiece. The dopant ions physically bombard and enter the surface of the workpiece, and subsequently come to rest below the workpiece surface in the crystalline lattice structure thereof.
Ion implantation has become the technology preferred by industry to dope semiconductors with impurities in the large-scale manufacture of integrated circuits. Ion dose and ion energy are the two most important variables used to define an implant step. Ion dose relates to the concentration of implanted ions for a given semiconductor material. Typically, high current implanters (generally greater than 10 milliamperes (mA) ion beam current) are used for high dose implants, while medium current implanters (generally capable of up to about 10 mA beam current) are used for lower dose applications.
Ion energy is the dominant parameter used to control junction depth in semiconductor devices. The energy levels of the ions that make up the ion beam determine the degree of depth of the implanted ions. High energy processes such as those used to form retrograde wells in semiconductor devices require implants of up to a few million electron-volts (MeV), while shallow junctions may demand ultra low energy (ULE) levels below one thousand electron-volts (1 keV).
A typical ion implanter comprises three sections or subsystems: (i) an ion source for generating an ion beam, (ii) an ion beam extraction system, (iii) a beamline including a mass analysis magnet for mass resolving the ion beam, and (iv) a target chamber which contains the semiconductor wafer or other substrate to be implanted by the ion beam. The continuing trend toward smaller and smaller semiconductor devices is driving beamline constructions to deliver high beam currents at low energies. High beam currents provide the desired dosage levels, while low energies permit shallow implants. Source/drain extensions in CMOS devices, for example, make it desirable for such a high current, low energy application.
Ion sources in ion implanters typically generate an ion beam by ionizing a source gas containing a desired dopant element within an ion source chamber, and an extraction system extracts the ionized source gas in the form of an ion beam. The ionization process is effected by an electron beam, which may take the form of a thermionic emitter such as a thermally heated filament, or a radio frequency (RF) antenna. A thermionic emitter is typically electrically biased so that emitted electrons gain sufficient energy to ionize, while an RF antenna delivers a high energy RF signal into the source chamber to energize ambient electrons.
The high-energy electrons thus ionize the source gas in the ion source chamber to generate desired ions. Examples of desired dopant ions produced from the source gas may include boron (B), phosphorous (P) or arsenic (As). In an ion source utilizing a thermionic emitter for ionization, the local emitter temperature typically exceeds 2500° C., and the source chamber being thermally irradiated by the emitter may attain temperatures on the order of 700° C.
Ions generated within the ion source are extracted through an elongated source aperture or slit by an electric field associated with one or more extraction electrodes located outside of the source chamber. The source aperture and the extraction electrodes may be made of graphite, taking advantage of the low vapor pressure of graphite at high temperatures and the reduced contamination risk to the workpiece, since very small levels of carbon in silicon have small effects on the semiconductor's electrical properties. Each extraction electrode system typically comprises spaced-apart elements forming an elongated extraction gap through which the ion beam travels. If a positively charged ion beam is desired, the extraction electrode is electrically biased negatively with respect to the source aperture.
Typically, for positive ion extraction more than one extraction electrode are used, with one electrode acting as electron suppression electrode by providing a barrier for electrons present in the system downstream of the extraction system. The suppression electrode is therefore biased negatively with respect to the beamline potential, and the last electrode in the extraction system is typically at beamline potential, to prevent the electric fields from the extraction system from affecting beam transport after extraction. A typical extraction system thus comprises two electrodes; if more electrodes are used, the magnitude of the voltages on the plurality of electrodes is typically decreased on each successive electrode moving downstream so as to provide an accelerating field for the positive ion beam, until the suppression bias is reached.
In designing such an ion implanter, it is desirable for the ion beam generated by and extracted from the ion source to accurately follow a desired predetermined travel path. The precise position of the extraction electrode with respect to the source aperture is important in achieving a beam path that coincides with the predetermined beam path. Thus, precise alignment and positioning of the extraction electrode or electrodes with the source aperture is typically desired.
Extraction electrodes are commonly mounted on a structure that extends from and/or is connected to the source housing. Heat generated by the operation of the ion source during ion implantation processes often causes thermal expansion of this structure, thus resulting in misalignment of the extraction electrodes with the source aperture when the temperature of the electrode system varies while the system reaches a new equilibrium. Further, alignment of the electrodes with respect to the ion source conventionally necessitates removal of the ion source and/or extraction electrodes from the ion implantation system, whereby various deleterious alignment issues can arise. Alternatively, the extraction electrodes are separate from the ion source, whereby removal of either the ion source or the extraction electrodes for maintenance can lead to additional misalignment. Such misalignments may cause unwanted disruptions in the intended path of the ion beam and result in unwanted “beam steering”, as well as distortions in the ion beam quality which could impair its transport through the rest of the beamline.
Various mechanisms for adjusting the position of the extraction electrodes with respect to the source aperture in ion implanters are known. Such mechanisms can be seen in U.S. Pat. No. 5,420,415 to Trueira, U.S. Pat. No. 5,661,308 to Benveniste et al., and U.S. Patent Publication No. 2005/0242293 to Benveniste. Conventionally, manipulation of the extraction electrodes is performed by extraction electrode manipulators that are separate from the ion generation mechanism or ion source. As such, multiple components are removed from the ion implantation system, thus leading to additional misalignment.
Additionally, the extraction electrodes are conventionally electrically isolated from the ion source in order to provide proper extraction potentials between the extraction electrode and ion source. Separation of the extraction electrode and ion source has been achieved by using vacuum as an electrical insulator, as doing so makes the individual components easier to handle and maintain. However, using vacuum as the electrical insulator comes at a disadvantage of mechanical decoupling, thus leading to alignment issues as described above. Conventionally, in order to service either the ion source or the extraction electrodes, an operator would remove the two assemblies separately, and then realign them with fixtures. Thus, conventional ion source and extraction electrode maintenance has been time-consuming and often fraught with misalignment issues that lead to additional downtime or waste.